1. Field of the Invention
The present invention concerns a transmission unit for motor vehicles, in particular a transmission unit of the type which is provided with a continuously variable transmission, a reversing unit and a number of clutches to engage the reversing unit in one or the other sense of rotation, whereby at least one of these clutches is controlled by a hydraulic pressure unit, with a piston which can be moved in a cylinder, whereby the seal between the piston and the cylinder comprises of an elastic sealing ring.
2. Discussion of the Prior Art
The above-mentioned clutches usually contain a forward clutch and a reverse clutch. The use of these clutches has a number of disadvantages, which will be explained hereafter on the basis of the existing situation.
When for example pressure is exerted on the piston of the forward clutch, the forward clutch is closed and the space behind the piston is, logically, entirely filled with hydraulic medium under pressure. In order to make sure that, when the pressure is relieved again, the piston can return to its initial position under the influence of a spring, there is a little tap hole in the piston, also called blow-off hole, such that the hydraulic medium can leave the above-mentioned space. A certain amount of hydraulic medium always remains behind the piston.
In the existing embodiments however, this tap hole is not situated on the largest diameter of the piston. If one subsequently drives in reverse, the shaft and the piston will also rotate, and a centrifugal pressure is built up behind the piston of the forward clutch which makes sure that the forward clutch is closed. Naturally, this causes losses, and since the forward clutch is usually not cooled in this phase, it can even burn.
Due to practical reasons, however, moving the conventional tap hole to a larger diameter is not easy. In order to avoid a shock and yet be able to use a large diameter, several holes are preferably provided along the perimeter, whose total surface, in order to restrict leakage losses, is preferably not larger than the surface of one conventional tap hole. Consequently, these holes have such a small diameter that it is very expensive to provide them in the metal piston.